Testing the local diachroneity of the terrestrial lithostratigraphic KPg boundary, Northern Montana

Abstract:

The lithostratigraphic KPg boundary separating the underlying Cretaceous Hell Creek Formation from overlying strata of the Paleogene Fort Union Formation in north-central Montana is delineated at the lowermost coal (Z-coal) at the base of the Fort Union. Since meandering stream floodplain coals record deposition in areally-restricted, meanderbelt environments, the lithostratigraphic KPg boundary is diachronous. To test this assertion, a chronostratigraphic framework, developed using radiometrically dated (40 Ar-39 Ar) ash horizons, is utilized to examine the physical stratigraphic relations between strata of the uppermost Hell Creek and lowermost Fort Union Formations. The uppermost (3m) Hell Creek Formation consists of massive mudrock with sporadic coal stringers. The lowermost (3m) Fort Union Formation consists of massive mudrocks with the discontinuous Z-coal at its base. No sedimentary structures are present. Three ash horizons, distinguished in the field by color and mineralogy, were used to establish correlations. The lower and middle ashes have been dated at 65.00 ± 0.05 Ma and 64.95 ± 0.05 Ma, respectively (Swisher et al., 1993). Six sites are studied. From east to west, these are: Mossbrucker, Lerbekmo, Pearl, Bone Hollow, Hell's Hollow, and Nirvana. Each site was trenched 3 meters above and below the Z-coal and lithofacies described every centimeter. The lithostratigraphic boundary at Lerbekmo is 55cm below the chronostratigraphic framework, 56 cm below at Mossbrucker, 9 cm above at Pearl, 258 cm below at Nirvana, and was not preserved at Bone Hollow and Hell's Hollow. Wheeler diagrams of these sections suggest the Z-coal is stratigraphically lower in the west and higher in the eastern sections. These observations demonstrate that the lithostratigraphic KPg boundary is diachronous implying portions of the Hell Creek are Paleogene in age and portions of the Fort Union Cretaceous in age. Furthermore, this demonstrates that lithostratigraphy provides insufficient constraints on determining chronostratigraphic position. Localized lithostratigraphic correlations of the terrestrial sections are also complicated by subtle diastems. Since floodplain deposition is centralized in the floodplain trough (Pizzuto et al., 2008) and a single flood event has limited areal extent (Aalto et al., 2003), the precision of lithostratigraphic correlations is limited without a chronostratigraphic framework of isochronous event beds.